I've actually run into this one myself, but I haven't tracked down the cause unfortunately. Can you tell me what version of the Blackberry OS you're using?

Right now, there's no way to get around it without rebooting your blackberry (pulling the battery). Once it's rebooted it'll start working fine again, not that it's an optimal solution. I have an idea where the issue might lie, but it happens pretty infrequently so I haven't looked into it yet.